glMemoryBarrier — defines a barrier ordering memory transactions
void glMemoryBarrier(GLbitfield barriers);
barriers
Specifies the barriers to insert. Must be a bitwise combination of GL_VERTEX_ATTRIB_ARRAY_BARRIER_BIT,
GL_ELEMENT_ARRAY_BARRIER_BIT, GL_UNIFORM_BARRIER_BIT, GL_TEXTURE_FETCH_BARRIER_BIT,
GL_SHADER_IMAGE_ACCESS_BARRIER_BIT, GL_COMMAND_BARRIER_BIT, GL_PIXEL_BUFFER_BARRIER_BIT,
GL_TEXTURE_UPDATE_BARRIER_BIT, GL_BUFFER_UPDATE_BARRIER_BIT,
GL_FRAMEBUFFER_BARRIER_BIT, GL_TRANSFORM_FEEDBACK_BARRIER_BIT, GL_ATOMIC_COUNTER_BARRIER_BIT,
or GL_SHADER_STORAGE_BARRIER_BIT.
If the special value GL_ALL_BARRIER_BITS is specified, all supported barriers will be inserted.
glMemoryBarrier defines a barrier ordering the memory transactions issued prior to the
command relative to those issued after the barrier. For the purposes of
this ordering, memory transactions performed by shaders are considered to
be issued by the rendering command that triggered the execution of the
shader. barriers is a bitfield indicating the set of operations that
are synchronized with shader stores; the bits used in barriers are as
follows:
GL_VERTEX_ATTRIB_ARRAY_BARRIER_BIT
If set, vertex data sourced from
buffer objects after the barrier will reflect data written by shaders
prior to the barrier. The set of buffer objects affected by this bit
is derived from the buffer object bindings used for
generic vertex attributes derived from the GL_VERTEX_ATTRIB_ARRAY_BUFFER bindings.
GL_ELEMENT_ARRAY_BARRIER_BIT
If set, vertex array indices sourced from
buffer objects after the barrier will reflect data written by shaders
prior to the barrier. The buffer objects affected by this bit are
derived from the GL_ELEMENT_ARRAY_BUFFER binding.
GL_UNIFORM_BARRIER_BITShader uniforms sourced from buffer objects after the barrier will reflect data written by shaders prior to the barrier.
GL_TEXTURE_FETCH_BARRIER_BITTexture fetches from shaders, including fetches from buffer object memory via buffer textures, after the barrier will reflect data written by shaders prior to the barrier.
GL_SHADER_IMAGE_ACCESS_BARRIER_BITMemory accesses using shader image load, store, and atomic built-in functions issued after the barrier will reflect data written by shaders prior to the barrier. Additionally, image stores and atomics issued after the barrier will not execute until all memory accesses (e.g., loads, stores, texture fetches, vertex fetches) initiated prior to the barrier complete.
GL_COMMAND_BARRIER_BIT
Command data sourced from buffer objects by
Draw*Indirect commands after the barrier will reflect data written by
shaders prior to the barrier. The buffer objects affected by this bit
are derived from the GL_DRAW_INDIRECT_BUFFER binding.
GL_PIXEL_BUFFER_BARRIER_BIT
Reads and writes of buffer objects via the
GL_PIXEL_PACK_BUFFER and GL_PIXEL_UNPACK_BUFFER
bindings (via glReadPixels,
glTexSubImage, etc.) after the
barrier will reflect data written by shaders prior to the barrier.
Additionally, buffer object writes issued after the barrier will wait
on the completion of all shader writes initiated prior to the barrier.
GL_TEXTURE_UPDATE_BARRIER_BIT
Writes to a texture via glTex(Sub)Image*,
glCopyTex(Sub)Image*, glCompressedTex(Sub)Image*, and reads via
glGetTexImage after the barrier will reflect data written by shaders
prior to the barrier. Additionally, texture writes from these
commands issued after the barrier will not execute until all shader
writes initiated prior to the barrier complete.
GL_BUFFER_UPDATE_BARRIER_BITReads or writes via glBufferSubData, glCopyBufferSubData, or glGetBufferSubData, or to buffer object memory mapped by glMapBuffer or glMapBufferRange after the barrier will reflect data written by shaders prior to the barrier. Additionally, writes via these commands issued after the barrier will wait on the completion of any shader writes to the same memory initiated prior to the barrier.
GL_FRAMEBUFFER_BARRIER_BITReads and writes via framebuffer object attachments after the barrier will reflect data written by shaders prior to the barrier. Additionally, framebuffer writes issued after the barrier will wait on the completion of all shader writes issued prior to the barrier.
GL_TRANSFORM_FEEDBACK_BARRIER_BITWrites via transform feedback bindings after the barrier will reflect data written by shaders prior to the barrier. Additionally, transform feedback writes issued after the barrier will wait on the completion of all shader writes issued prior to the barrier.
GL_ATOMIC_COUNTER_BARRIER_BITAccesses to atomic counters after the barrier will reflect writes prior to the barrier.
GL_SHADER_STORAGE_BARRIER_BITAccesses to shader storage blocks after the barrier will reflect writes prior to the barrier.
If barriers is GL_ALL_BARRIER_BITS, shader memory accesses
will be synchronized relative to all the operations described above.
Implementations may cache buffer object and texture image memory that
could be written by shaders in multiple caches; for example, there may be
separate caches for texture, vertex fetching, and one or more caches for
shader memory accesses. Implementations are not required to keep these
caches coherent with shader memory writes. Stores issued by one
invocation may not be immediately observable by other pipeline stages or
other shader invocations because the value stored may remain in a cache
local to the processor executing the store, or because data overwritten by
the store is still in a cache elsewhere in the system. When glMemoryBarrier
is called, the GL flushes and/or invalidates any caches relevant to the
operations specified by the barriers parameter to ensure consistent
ordering of operations across the barrier.
To allow for independent shader invocations to communicate by reads and
writes to a common memory address, image variables in the OpenGL Shading
Language may be declared as "coherent". Buffer object or texture image
memory accessed through such variables may be cached only if caches are
automatically updated due to stores issued by any other shader invocation.
If the same address is accessed using both coherent and non-coherent
variables, the accesses using variables declared as coherent will observe
the results stored using coherent variables in other invocations. Using
variables declared as "coherent" guarantees only that the results of
stores will be immediately visible to shader invocations using
similarly-declared variables; calling glMemoryBarrier is required to ensure
that the stores are visible to other operations.
The following guidelines may be helpful in choosing when to use coherent memory accesses and when to use barriers.
Data that are read-only or constant may be accessed without using coherent variables or calling MemoryBarrier(). Updates to the read-only data via API calls such as BufferSubData will invalidate shader caches implicitly as required.
Data that are shared between shader invocations at a fine granularity (e.g., written by one invocation, consumed by another invocation) should use coherent variables to read and write the shared data.
Data written by one shader invocation and consumed by other shader invocations launched as a result of its execution ("dependent invocations") should use coherent variables in the producing shader invocation and call memoryBarrier() after the last write. The consuming shader invocation should also use coherent variables.
Data written to image variables in one rendering pass and read by the
shader in a later pass need not use coherent variables or
memoryBarrier(). Calling MemoryBarrier() with the
SHADER_IMAGE_ACCESS_BARRIER_BIT set in barriers between passes is
necessary.
Data written by the shader in one rendering pass and read by another
mechanism (e.g., vertex or index buffer pulling) in a later pass need
not use coherent variables or memoryBarrier(). Calling
glMemoryBarrier with the appropriate bits set in barriers between
passes is necessary.
glMemoryBarrier is available only if the GL version is 4.2 or higher.
GL_SHADER_STORAGE_BARRIER_BIT is available only if the GL version is 4.3 or higher.
GL_INVALID_VALUE is generated if barriers contains any bits
other than those listed above, or is not the special value GL_ALL_BARRIER_BITS.
Copyright © 2011 Khronos Group. This material may be distributed subject to the terms and conditions set forth in the Open Publication License, v 1.0, 8 June 1999. http://opencontent.org/openpub/.